1. Field of the Invention:
This invention relates to an improved method for the production of paperboard by the combined use of press-drying and ultrasonic energy to enhance the removal of moisture from and increase the density of the paperboard.
2. Description of the Prior Art:
A prior art search directed to the subject matter of this application in the U.S. Patent and Trademark Office disclosed the following U.S. Pat. Nos.: 838,616; 1,347,723; 1,479,265; 2,122,376; 2,135,763; 2,209,761; 2,624,245; 2,907,690; 3,057,075; 3,148,108; 3,162,568; 3,319,352; 3,354,035; 3,515,633; 3,655,507; 3,691,010; 3,799,052; 3,973,483; 4,011,034; 4,040,899; 4,139,410; 4,163,688; 4,324,613; 4,351,700; 4,384,514; 4,391,670; 4,506,456.
None of the prior art patents uncovered in the search discloses the simultaneous use of press-drying and ultrasonic energy to form paperboard.